• Gas exchange, in concert with the circulatory system,provide the oxygen necessary for aerobic cellular respirationand removes the waste product, carbon dioxide.

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Fig. 42.18

Requirements for Gas Exchangemoist membranesurface arearespiratory medium

Air Water

• In some invertebrates, such as sea stars, the gills have asimple shape and are distributed over much of the body.

• Many segmented wormshave flaplike gills thatextend from eachbody segment, or longfeathery gills clusteredat the head or tail.

• The gills of clams,crayfish, and manyother animals arerestricted to a localbody region.

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Fig. 42.19

Gills

Ventilation, which increases the flow of the respiratory medium over the respiratory surface, ensures that there is a strong diffusion gradient between the gill surface and the environment.

Crayfish and lobsters have paddlelike appendages that drive a current of water over their gills.

Fish gills are ventilated by a current of water that enters the mouth, passes through slits in the pharynx, flows over the gills, and exits the body.

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Fig. 42.20

Ventilation - gill cover

• This flow pattern is countercurrent exchange.

• As blood moves anteriorly in a gill capillary, it becomesmore and more loaded with oxygen, but itsimultaneously encounters water with even higheroxygen concentrations because it is just beginning itspassage over the gills.

• All along the gillcapillary, there is adiffusion gradientfavoring the transferof oxygen fromwater to blood.

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Fig. 42.20

Terrestrial organismsGills unsuited for an animal living on land.

Air- respiratory medium

Advantages of air over waterhigher concentration of oxygen.O2 and CO2 diffuse much faster in air less energy is needed to ventilate

Disadvantages of air as a respiratory mediumloss of water

Terrestrialrespiratory surfaces within the bodyopening to the atmosphere through narrow tubes.

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Fig. 42.22

Insects - tracheal system TracheaGas exchange at cell by diffusion across the moist epithelium

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Fig. 42.23

LungsAmphibians lungs & skinReptiles, Birds & Mammals - only lungs

• In contrast, mammals ventilate their lungs bynegative pressure breathing.

• This works like a suction pump, pulling air instead ofpushing it into the lungs.

• Muscle action changes the volume of the rib cage and thechest cavity,and the lungsfollow suit.

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Fig. 42.24

Ventilation - (breathing) Inhalation Exhalation

Amphibians - Positive pressure

• Ventilation is much more complex in birds than inmammals.

• Besides lungs, birds have eight or nine air sacs that donot function directly in gas exchange, but act as bellowsthat keep air flowing through the lungs.

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Fig. 42.25

Lung Volumes - Residual Volume differences